1. Field of the Invention
This invention relates to a method of manufacturing foundry sand molds (including cores). More particularly, it relates to an improved method of manufacturing sand molds which comprises mixing (a) a carboxylic acid, (b) a divalent or trivalent metal oxide or hydroxide and (c) a foundry molding sand in the presence of and aqueous solution of (d) a carboxyl-group-containing polymer or water (e) followed by shaping and hardening.
2. Description of the Prior Art
Foundry sand molds have been manufactured by adding a variety of inorganic or organic binders to a foundry sand. However, each of these inorganic and organic binders has its own merits and demerits. Thus they are not always satisfactory. When water glass, for instance, is used as a binder, the molding sand can be used only once and disposal of the used sand may result in gradual dissolution of the water glass adhering to the sand surface, which means leaching of alkaline substances and consequent alkalinization of the surrounding soil, leading not only the withering of trees and plants but also to killing of organisms living in the soil and in the water. When organic binders such as self-curing furan or phenol resins are used, the organic materials can be removed after use by burning the sand and the sand can be reused. However, in burning the sand, a bad smell is produced. Moreover, the rate of curing of the binder and the mechanical strengths of the sand mold vary significantly depending on such factors as properties of the sand, moisture content thereof, temperature, humidity, kind of acid used as binder and concentration thereof and consequently handling of the molding sand is a task of considerable difficulty. It is also a disadvantage that such a binder must be used up within several months after preparation because polymerization of the binder proceeds gradually during storage thereof.
According to the method disclosed in Japanese Patent Publication No. 6259/1953 or U.S. Pat. No. 2,765,507, which uses an acrylic acid polymer as a binder or according to the method disclosed in Japanese Patent Publication No. 8708/1961 (Patent of Addition to the above), which uses an acrylic acid-acrylonitrile copolymer as a binder, no unpleasant smell is produced in the manufacture of sand molds or in the pouring of molten metal, but green strengths are very low, whereby roasting is required for practical use. In the present time when extra-large castings are manufactured, it is difficult to prepare an apparatus for such roasting and therefore such binders cannot be employed in foundry practice. Although it is known that green strength can be increased by adding such metal oxides and hydroxides as magnesium oxide and aluminum hydroxide as curing agents for the binders used according to the above-mentioned method, mere standing of the sand mold in atmospheric conditions without roasting thereof after manufacture can indeed lead to easy hardening of the surface portion but the inner part does not harden so that only very low green strength is obtained. The resulting sand mold therefore cannot endure the load to be carried thereby in pouring molten metal in the manufacture of relatively large castings. In addition, where there is no exposure to the atmosphere, the moisture originating from the aqueous solution of the polymer used as a binder remains in the sand mold. On pouring molten metal, the moisture in the molding sand, i.e. the mold, is converted into water vapor by the high temperature of the molten metal. The vapor blows out and causes formation of uneven surfaces of the casting. It is also a disadvantage that castings having smooth surfaces cannot be obtained.
Another method of manufacturing foundry sand molds is disclosed in Japanese Patent Application Laid-Open No. 129,121/1978 or German Patent Application Laid-Open (Offenlegungsschrift) No. 2,815,753. The method comprises adding polyacrylic acid, aluminosilcate glass, tartaric acid and/or molten granular magnesia to a molding sand followed by mixing in the presence of water and shaping. However, this method has some disadvantages. Thus, since the principal component of the curing agent is the aluminosilicate glass, crushing after casting is troublesome and accumulation of the glass caused by repeated use may make further reuse a hard task. Furthermore, the moisture content in the sand mold as manufactured amounts to 2 to 5%, which is far above the preferable moisture content of not more than about 1%.
Furthermore, the method to make foundry sand mold by kneading the foundry sand adding citric acid, metal oxide and water is known in the Japanese Patent Application No. 27404/1976. This method has the disadvantages of shortening the working time, becoming rapidly sticky when an ordinary metal oxide is used, and becoming incapable of practical use because of the remarkably low residual green strength at high temperature. In addition, a method using a combination of metal oxides such as zinc oxide, hydroxylic acids such as tartaric acid and water as binder components is also disclosed in the Japanese Patent Application Laid-Open No. 126326. However, this method has the same disadvantages as mentioned above.